[llvm] [DirectX][DXIL] Design document for TableGen Spec of DXIL Operations (PR #85170)

[Helena Kotas via llvm-commits]

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+==============================================================
+Specification of DXIL Operations using TableGen Representation
+==============================================================
+.. contents::
+   :local:
+
+.. toctree
+   :hidden
+
+Introduction
+============
+
+`DirectXShaderCompiler <https://github.com/microsoft/DirectXShaderCompiler>`_
+encapsulates, among other information, various DXIL Operations in
+`hctdb.py <https://github.com/microsoft/DirectXShaderCompiler/blob/main/utils/hct/hctdb.py>`_.
+DXIL Operations are represented in one of the following `two ways
+<https://github.com/microsoft/DirectXShaderCompiler/blob/130877392c263888ef06bab768856d3dab1f1c9a/docs/DXIL.rst#L1978>`_:
+
+#. Using LLVM instructions
+#. Using LLVM External functions. These are represented in LLVM IR as follows:
+
+  * "Standard" LLVM intrinsics (e.g., ``llvm.sin.*``) and
+  * HLSL intrinsics (defined as LLVM intrinsics in ``llvm/include/llvm/IR/IntrinsicsDirectX.td``, e.g., ``llvm.dx.*``)
+
+  These are  collectively referred to as `LLVM Intrinsics` in this note.
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hekota wrote:

The rendering still seems off. Have you considered merging the two lists together?

#. Using LLVM instructions (standard LLVM intrinsics, e.g., ``llvm.sin.*``)
#. Using LLVM External functions, also called HLSL intrinsics. These are defined in ``llvm/include/llvm/IR/IntrinsicsDirectX.td`` (e.g., ``llvm.dx.*``).

https://github.com/llvm/llvm-project/pull/85170